Protractor



(No Model.)

P. JOHNSON. PROTRAGTOR..

No. 418,133. y Patented Dec. 24, 1889.

N. PETERS. Phulmmhogmbhnr. wa'shngmn. D. t;

UNITED STATES l PATENT OFFICE.

FROST JOHNSON, OF NEWYO-RK, N. Y., ASSIGNOR OF TVO-TIIIRDS TO ELISHA G.SELCHOV, OF SAME PLACE. AND VILLIAM H. WALKER, OF ALBANY,

' NEV YORK.

PROTRACTO R.

SPECIFICATION forming part of Letters Patent No. 418,133, dated December24, 1889. Application iiled April 11, 1889- Serial No. 306,877. (Nomodel.)

To @ZZ whom, t may concern:

Be it known that I, FROST JOHNSON, a citizen of the United States,residing at New York, in the county of New York and State of New York,have invented new and useful Improvements in Protractors, of which thefollowing is a specification.

My invention relates to certain improvements in that class ofmathematical instruments commonly known as protractors, and it consistsof a semi-annular metallic plate graduated to show one hundred andeighty degrees.

It is the purpose of my invention to provide a novel and simpleinstrument of this class, whereby the graduated arc may be placed insuch position that a true vertical line will pass through the centralline of graduation indicating ninety degrees, and to combine with theprotractor a straight edge concentrically pivoted thereon and adapted tosweep over the entire circle of graduation, whereby the operator maydetermine the angle of inclination of any object and reproduce orrepresent the same upon paper or any other material.

The invention consists in the several novel features of construction andnew combinations Of parts hereinafter fully set forth, and pointed outin the claims.

Referring to the accompanying drawings, Figure l is a plan view showingmy invention. Fig. 2 is a rear elevation of the same. Fig. 3 is a detailsection.

In the said drawings, the reference-nu meral l denotes a brassprotractor-plate extending over somewhat more than half of acircle andgraduated in the ordinary manner. Upon the arms 2, connecting the endsof the semicircular plate, is formed a central plate 3, which serves asa handle, said plate being formed, preferably, by curving the ends ofthe arms 2 downward, leaving a central longitudinal opening 4, belowwhich the said plate extends a suitable distance. Upon a bracket 5projecting from the arms 2 is mounted a pivot 6, which is central to thecircle of graduation, and upon said pivot is mounted'a straight-edge arm7, which projects beyond the outer edge of the protractor-plate. Thestraight edge 8 of this arm is brought into the axial line of the pivot,and in order to aid the eye in making correct adjustments a notch is cutin said edge directly over the protractor-plate, forming a sharp point9, which is in the line of the edge S, and assists in setting the arm atany point in the graduated circle. Upon t-he rear face of the bracket 5is pivoted a wire or bar l0, carrying on its lower end a weight 12, andforming a pendulum which will by gravity occupy a true verticalposition. The swinging ofthe pendulum is limited by a keeper-bar 13, andits pivotpoint is in a line drawn directly through the pivotal axis 6,and through the point graduated to ninety degrees on the limb l. Aline14 is also drawn on the solid part of the plate 3, terminating in apoint l5, which is in a continuation of the line passing through thepivotal axis and the point of suspension of the pendulum. A hair wire i6may also be stretched from the pivotal axis to the central point ofgraduation of ninety degrees. The circle of graduation is extended overthree hundred and sixty degrees by lines upon the inferior curves formedby the arms 2.

The manner of using the device is apparent. The operator holding theplate in such manner that the pendulum is free t-o move brings thelatter to coincide with the central point l5. The arm 7 is then turneduntil its straight edge coincides with the outline of the object underexamination, after which the angle thus ascertained may be transferredto paper, as described.

Having thus described my invention, what I claim isn l. Aprotractor-plate having at its ends arms forming inferior curves, andgraduated in continuance of the graduations of theplate, a plumbpivotally mounted in the central line of thel protractor, and astraight-edge arm pivoted in said line and adapted to sweep thegraduated surface of the plate, substantially as described.

2. A prOtractor-plate having arms at its ends forming inferior curvesand graduated in continuance of the graduations of the plate, a plumbpivotally mounted on a bracket on the arms, and a straight-edge armpivoted on said bracket and having its end projecting over the graduatedsurface, a notch formed TOO therein to leave a sharp point coincidentwith the straight edge, substantially as described.

3. A protraetor having graduated arms forming inferior curves andleaving a slot or opening between them, a plumb pivoted above saidopening and having its weight swinging below said opening, astraight-edge arm pivoted on a central bracket and sweeping thegraduated surface, and a hair-Wire stretched in a line passing throughthe pointpf ninety degrees and the pivotal axis of the arm,substantially as described.

4. A protractor-plate having a plumb pivoted in a central line andprovided with a central handle having an opening through 4tially asdescribed.

which the pendulum-bar is disclosed, substan- 5. A practractor-platehaving a central handle and a fixed radius, formed of a hair-wire, incombination with a plumb pivoted in a line coinciding with said radiusand displayed through an opening in the central handle, substantially asdescribed. y

In testimony whereof l have affixed my signature in presence of twowitnesses.

p FROST JOHNSON. Witnesses:

PEROY B. HILLS,

JAMES A. RUTHERFORD.

